Cu-alloy plaque
Object property of the Museum of the Order of St John
Object before treatment
Object after treatment
Condition before treatment
The plaque has four holes on each of its edges with one remaining screw on the upper right side. There are corrosion products created from dripping moisture from the top two holes where voluminous corrosion products have formed. There is no indication of active corrosion, the products are carbonates and chlorides. There are remains of what looks like paint splattered throughout the surface.
A stable patina introduced by chloride ions is present throughout the surface. It is concentrated towards the middle of the plaque. What appears to be a previous cleaning attempt with a material that was not properly removed from the surface post-cleaning has etched the surface in a characteristic pattern.
The rear of the plaque has copper corrosion products formed towards the four edges. The three washers on the back are in good condition.
Treatment
*The surface was degreased after each step as appropriate with IMS (industrial methylated spirits)
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Corrosion products were removed mechanically with a scalpel and the Dremel tool.
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The patina on the surface was removed with sodium dithionite (see Image 9) and rinsed with IMS.
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The surface was polished with Brasso (metal polish) and a soft cloth.
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A protective coating of Renaissance Wax (microcrystalline wax) was applied on the front and rear surfaces. Areas of severe pitting induced by the corrosion products were initially covered with a layer of 5% w/v Paraloid B44 (ethyl methacrylate copolymer) in acetone. The whole surface was covered with a layer of Renaissance Wax (microcrystalline wax) applied with a brush and buffered with a soft cloth.
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Losses of inlaid decoration were filled with 20% w/v Paraloid B72 in acetone with phenolic microballoons (glass microspheres) pigmented with mineral pigments.
Treatment justifications
Patination was removed after consultation with owner. The removal of patination achieved a result close to other plaques previously conserved at the Crypt
Inlay fills with Paraloid B72 and microballoons: Paraloid B72 is a strong and suitable material when used with microballoons for small fills.
Use of mineral pigments: they will not introduce acids on the surface.
Coatings of microcrystalline wax to passivate the object and protect against atmospheric pollution and moisture.
